1. Field of the Invention
The invention pertains to an organic light-emitting device, and more particularly, to an organic light-emitting device that prevents a stripe pattern caused by device irregularities and a power voltage drop, and improves the aperture ratio.
2. Description of the Related Art
Generally, an organic light-emitting device is a self-emissive display device that emits light by electrically exciting a luminous organic compound. The organic light-emitting device can drive an N×M number of organic light-emitting diodes (OLEDs) to display an image.
Driving the organic light-emitting device occurs in a passive matrix manner or in an active matrix manner using a transistor. The organic light-emitting device using the passive matrix manner is driven with an anode vertical to a cathode and a selection line. In comparison to the passive matrix driving, the organic light-emitting device using the active matrix is driven with a transistor and a condenser connected to each ITO (indium tin oxide) pixel electrode to maintain a voltage by the condenser capacitance.
FIG. 1 illustrates a pixel of a related art active matrix organic light-emitting device, and typically illustrates one of the N×M pixels.
The related art active matrix organic light-emitting device of FIG. 1 includes a second transistor M2 being connected to the organic light-emitting diode (OLED) to supply current for luminescence, and the amount of current in the second transistor M2 is controlled by a mth data voltage (Data[m]) applied through a first transistor M1. A condenser C1 connects between a source electrode and a gate electrode of the second transistor M2 to maintain the applied Mth data voltage for a predetermined period. A gate line connects to the gate electrode of the first transistor M1 to supply an nth selection signal (Select[n]), and a data line is connected to the source electrode to supply an mth data voltage (Data[m]).
The operation of the above organic light-emitting device is described as follows. If the first transistor M1 is turned on by the nth selection signal (Select[n]) applied to a gate electrode of the first transistor M1, the mth data voltage (Data[m]) is applied to the gate electrode (node A) of the second transistor M2. Accordingly, the organic light-emitting diode (OLED) emits light by the driving current provided through the second transistor M2. That is, after the nth selection signal (Select[m]) is used to select a desired pixel, the organic light-emitting diode (OLED) emits light by the driving current flowing from the second transistor M2 generated by the applied mth data voltage (Data[m]).
The above-described organic light-emitting device is manufactured through the process shown in FIG. 2. As shown in FIG. 2, laser power outputted from an excimer laser is used to crystallize an amorphous silicon (a-Si) substrate into a polysilicon (p-Si) substrate. At this time, several variables determine the quality of the polysilicon. In particular, the polysilicon substrate has qualities sensitive to the laser power outputted from the excimer laser. That is, the excimer laser has unstable laser power strength depending on time. Accordingly, the crystallized polysilicon substrate has an unstable, i.e., variable, quality.
The amorphous substrate is crystallized into the polysilicon substrate by unidirectionally irradiating the laser power into the amorphous substrate (that is, using one scan direction). The polysilicon substrate has an irregular characteristic in the scan direction, but has a regular characteristic in a direction vertical to the scan direction.
If the polysilicon substrate has an irregular characteristic, a threshold voltage (Vth) of the manufactured driving transistor (for example, second transistor M2 of FIG. 1) becomes variable. Accordingly, the threshold voltages of the driving transistors provided at respective pixels are different from one another, thereby causing current flowing the driving transistors to be different from one another. As a result, there is a drawback in that the desired grayscale and uniformity cannot be obtained.
If the crystallized polysilicon substrate is driven irregularly, the displayed image has a stripe pattern as shown in FIG. 3. This is caused by the variation of the threshold voltage of the driving transistor due to the irregularity of the crystallized substrate.
In the meantime, organic light-emitting devices have been vigorously studied for large-area driving together with other flat panel display devices.
In this application, the power voltage (Vdd) is applied to each pixel. The power voltage is generally applied to a lower side from an upper side of a panel. The power voltage is applied along the power line. Since the power line has an internal line resistance, a power voltage lower than that of the upper side of the panel is applied at the lower side due to the voltage drop (IR-drop). Since the lower power voltage is applied at the lower side than the upper side of the panel due to the voltage drop (IR-drop), there is a drawback in that the driving current relating to the power voltage is reduced, thereby not providing the desired grayscale.